Inspection of powerlines is a hard problem that requires humans to operate in remote locations and dangerous conditions. This paper proposes a quadcopter unmanned aerial vehicle (UAV) equipped with rolling-capable perching mechanisms and a depth-vision system for the purpose of autonomous power line inspection. The perching mechanism grips onto the power line, allowing the UAV to withstand external forces such as wind disturbances. Once engaged and applying the desired gripping force, the perching mechanism requires no power through the use of a ratcheting serial elastic transmission, allowing the UAV to perch indefinitely. The depth-vision system automates the perching and unperching procedures by estimating the position and pose of the UAV relative to the powerline. These measurements are sent to a local position controller that guides the UAV to and from the power line. Once perched, rollers in the fingers of the perching mechanism drive the UAV along the powerline, providing a close-up platform for inspection equipment. The proposed system was tested in an outdoor testing environment and shown to autonomously perch and unperch from a steel cable. The grippers force application was analysed and the UAVs powerless robust perch is demonstrated by total disconnect of power while perched. These results suggest that such a system could be a valuable tool for the upkeep of electricity networks.